1. Field of the Invention
This invention relates to polymers of hexafluoropropene oxide (to be abbreviated as HFPO, hereinafter) and more particularly, to difunctional HFPO polymers having a narrow dispersity or molecular weight distribution. It also relates to a process for preparing the HFPO polymers.
2. Prior Art
Difunctional HFPO polymers are known in the art. For example, U.S. Pat. No. 3,250,807 discloses that difunctional HFPO polymers are prepared by reacting HFPO with FOC--(CF.sub.2).sub.n --COF wherein n is from 0 to 6, in an aprotic polar solvent in the presence of a catalyst such as an alkali metal fluoride or activated carbon according to the following reaction scheme. ##STR2##
Such an attempt to add HFPO to the preformed --COF groups often gives rise to the problem that a HFPO polymer having a hexafluoropropyl group only at one end (that is, monofunctional HFPO polymer) is formed as a by-product due to chain transfer reaction as shown below. ##STR3## Chain transfer ##STR4##
An improved process for producing difunctional HFPO polymers of high purity while preventing the chain transfer is disclosed in JP-B 5360/1978 and U.S. Pat. No. 3,660,315. According to the process disclosed therein, FOCCF(CF.sub.3)OCF.sub.2 CF.sub.2 OCF(CF.sub.3)COF is mixed with cesium fluoride in tetraethylene glycol dimethyl ether to form CsOCF.sub.2 CF(CF.sub.3)OCF.sub.2 CF.sub.2 OCF(CF.sub.3)CF.sub.2 OCs. The excess of cesium fluoride is separated from the solution to give a homogeneous solution. The homogeneous solution is used as a polymerization initiator for the preparation of HFPO polymer. Polymerization is carried out at low temperatures of -60.degree. C. to -30.degree. C. yielding a pure difunctional HFPO polymer having a number average degree of polymerization of about 50.
However, J. Macromol. Sci. Chem., A8 (3), 499 (1974) describes that if the molar ratio of HFPO to the initiator is increased in an attempt to produce a difunctional HFPO polymer having a high degree of polymerization, the formation of a monofunctional HFPO polymer by-product is increased and the purity of the difunctional HFPO polymer is reduced. JP-A 175185/1982 and U.S. Pat. No. 4,356,291 describe that a HFPO polymer having a number average degree of polymerization of 445 is obtained by using highly purified HFPO along with the above-described initiator although no reference is made to the formation of a monofunctional HFPO polymer by-product and the purity of the resulting difunctional HFPO polymer.
The prior art research works regarding difunctional HFPO polymers focused at the reduction of monofunctional HFPO polymer by-product resulting from chain transfer and the production of HFPO polymer having a high degree of polymerization.
It is now under development to form liquid rubber using difunctional HFPO polymer as the raw material as described in JP-A 77777/1997, 95615/1997, and 137027/1997. In this application, if a monofunctional HFPO polymer is admixed in the raw material, it gives detrimental influences such as the deteriorated physical properties of the resulting rubber and the hindrance of curing reaction. According to the inventor' finding, the most detrimental among these influences is that the dispersity of difunctional HFPO polymer as the raw material has a significant influence on the viscosity and flow properties of the compound and the physical properties of the cured rubber. If difunctional HFPO polymers as the raw material are different in dispersity, there results a significant difference in compound viscosity and cured rubber properties even though they have an identical degree of polymerization.
Accordingly, there is a desire to have a difunctional HFPO polymer having a narrow dispersity and a low content of monofunctional HFPO polymer so that it is suitable as a raw material for liquid rubber.